Many-to-many declarative SQLAlchemy definition for users, groups, and roles

Question

I'm new to SQLAlchemy, and was wondering what the best way to go about defining such tables and a relationship would be. I want to be able to access groups from a user by user.groups, users in a group by group.users, and find out the role of a user in a group (which I'm assuming will be logic defined in an association model). I also want to select all users, group by group, and include role titles.

I've tried using the tutorial to create the association table (declarative style) and removing the secondary argument to relationship properties in the User and Group classes, but then I would lose the ability to access groups directly from users, and users directly from groups (the backref's in the association table point to the association class itself :/).

Any help would be appreciated.

Answer 1

The key to simplifying your model is to use associationproxy, therefore you should definitely check it out.
Assuming that the user can have only one role within a group, the code below should answer all your questions:

• how to configure the model and relationships
• how to add/remove/update the roles
• how to retrieve the data (reports) you asked for

You should take over the model part and forget the rest. The completely contained and working script below:

from sqlalchemy import create_engine, Column, Integer, Unicode, ForeignKey
from sqlalchemy.orm import relationship, scoped_session, sessionmaker
from sqlalchemy.orm.collections import attribute_mapped_collection
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy

# Configure test data SA
engine = create_engine(u'sqlite:///:memory:', echo=False)
session = scoped_session(sessionmaker(bind=engine, autoflush=False))
Base = declarative_base()

class _BaseMixin(object):
    """
    A helper mixin class to set properties on object creation.

    Also provides a convenient default __repr__() function, but be aware that
    also relationships are printed, which might result in loading the relation
    objects from the database
    """
    def __init__(self, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)

    def __repr__(self):
        return "<%s(%s)>" % (self.__class__.__name__,
            ', '.join('%s=%r' % (k, self.__dict__[k])
                      for k in sorted(self.__dict__)
                      if '_' != k[0]
                      #if '_sa_' != k[:4] and '_backref_' != k[:9]
                      )
            )


# relation creator factory functions
def _creator_gr(group, role):
    res = UserGroup(group=group, role=role)
    return res
def _creator_ur(user, role):
    res = UserGroup(user=user, role=role)
    return res

##############################################################################
# Object Model
##############################################################################
class Role(Base, _BaseMixin):
    __tablename__ = 'roles'
    # columns
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(Unicode(16), unique=True)
    # relations
    usergroup = relationship("UserGroup", backref='role')

class User(Base, _BaseMixin):
    __tablename__ = 'users'
    # columns
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(Unicode(16), unique=True)
    # relations
    _rel_usergroup = relationship("UserGroup", collection_class=attribute_mapped_collection('group'),
                                  cascade='all,delete-orphan',
                                  backref='user',
                                  )
    groups = association_proxy('_rel_usergroup', 'role', creator=_creator_gr)

class Group(Base, _BaseMixin):
    __tablename__ = 'groups'
    # columns
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(Unicode(16), unique=True)
    # relations
    _rel_usergroup = relationship("UserGroup", collection_class=attribute_mapped_collection('user'),
                                  cascade='all,delete-orphan',
                                  backref='group',
                                  )
    users = association_proxy('_rel_usergroup', 'role', creator=_creator_ur)

class UserGroup(Base, _BaseMixin):
    __tablename__ = 'user_group'
    # columns
    id = Column(Integer, primary_key=True, autoincrement=True)
    user_id = Column(Integer, ForeignKey('users.id', ondelete='CASCADE'), nullable=False)
    group_id = Column(Integer, ForeignKey('groups.id', ondelete='CASCADE'), nullable=False)
    role_id = Column(Integer, ForeignKey('roles.id', ondelete='CASCADE'), nullable=False)
    # relations
    # (all backrefs)



##############################################################################
# TESTS (showing usages)
#
# Requirements:
#  - list all groups of the user: user.groups (use keys)
#  - list all users of the group: group.users (use keys)
#  - get all users ordered (grouped) by group with the role title
##############################################################################

def _requirement_get_user_groups(user):
    return user.groups.keys()

def _requirement_get_group_users(group):
    return group.users.keys()

def _requirement_get_all_users_by_group_with_role():
    qry = session.query(Group).order_by(Group.name)
    res = []
    for g in qry.all():
        for u, r in sorted(g.users.items()):
            value = (g.name, u.name, r.name)
            res.append(value)
    return res

def _test_all_requirements():
    print '--requirement: all-ordered:'
    for v in _requirement_get_all_users_by_group_with_role():
        print v

    print '--requirement: user-groups:'
    for v in session.query(User).order_by(User.id):
        print v, " has groups: ",  _requirement_get_user_groups(v)

    print '--requirement: group-users:'
    for v in session.query(Group).order_by(Group.id):
        print v, " has users: ",  _requirement_get_group_users(v)

# create db schema
Base.metadata.create_all(engine)

##############################################################################
# CREATE TEST DATA
##############################################################################

# create entities
u_peter = User(name='u_Peter')
u_sonja = User(name='u_Sonja')
g_sales = Group(name='g_Sales')
g_wales = Group(name='g_Wales')
r_super = Role(name='r_Super')
r_minor = Role(name='r_Minor')

# helper functions
def _get_entity(entity, name):
    return session.query(entity).filter_by(name=name).one()
def get_user(name):
    return _get_entity(User, name)
def get_group(name):
    return _get_entity(Group, name)
def _checkpoint():
    session.commit()
    session.expunge_all()
    _test_all_requirements()
    session.expunge_all()
    print '-' * 80


# test: **ADD**
u_peter.groups[g_wales] = r_minor # add
g_wales.users[u_sonja] = r_super # add
g_sales.users[u_peter] = r_minor # add
session.add(g_wales)
#session.add(g_sales)
_checkpoint()

# test: **UPDATE**
u_peter = get_user('u_Peter')
assert u_peter.name == 'u_Peter' and len(u_peter.groups) == 2
assert len(u_peter.groups) == 2
g_wales = get_group('g_Wales')
g_wales.users[u_peter] = r_super # update
_checkpoint()

# test: **DELETE**
u_peter = get_user('u_Peter')
assert u_peter.name == 'u_Peter' and len(u_peter.groups) == 2
g_wales = get_group('g_Wales')
del u_peter.groups[g_wales] # delete
_checkpoint()